Ligand-linked changes of ultrasound absorption of hemoglobin

The acoustic absorption of hemoglobin solutions at 1.88 MHz has been studied in dependence of ligand binding and pH. In the physiological pH range the excess absorption of oxyhemoglobin is increased compared to deoxyhemoglobin. Evidence is presented that in human hemoglobin the additional absorption of oxyhemoglobin is caused by a proton transfer relaxation process involving the alpha chain N-terminal alpha-amino groups. This process cannot be observed in deoxyhemoglobin because of fixation of the N-terminal alpha-amino groups in a salt bridge. Further investigations of carbonmonoxy, nitrosyl, aquomethemoglobin and fluoromethemoglobin in the absence and presence of inositol hexakissulfate show that acoustic absorption measurements offer an additional tool to characterize in greater detail the different conformational states of hemoglobin.